The RYA legal dept take on it is that if in familiar waters that you have travelled before, checking tides (especially for going round headlands etc) and weather is sufficient, no actual plan required to be written. If novices or heading into unfamiliar waters, then a great deal more planning would be required, preferably written.
The plan can be all in your head, and totally flexible, so you might end up going to a different destination if you find conditions on the day not pleasant etc. Straightforward common sense in effect